The aluminosilicate, i.e., zeolitic, molecular sieves as a general class of crystalline materials have been synthesized commercially for many years. Whereas the earlier species were hydrothermally crystallized from reaction systems containing only inorganic reagents, there now exist a considerable number of species prepared from reaction mixtures which contain organic bases either as templating agents, templating agent precursors or constituents which increase the pH conditions favorable for zeolite crystallization. The zeolite species synthesized using organic bases tend to have a wider range of framework Si/Al ratios which in certain instances exceed a value of several thousand, i.e., are essentially silica polymorphs. The framework structures can also contain tetrahedral or octahedral oxides of numerous other metals or metaloids, such as gallium, germanium, boron, vanadium, titanium and the like.
The aluminophosphate molecular sieves comprise a considerable number of crystalline compositions formed from PO.sub.2.sup.+ and AlO.sub.2.sup.- tetrahedral units only or in combination with SiO.sub.2 tetrahedra and/or one or more other structural metal oxide units which can be tetrahedral or octahedral or both. In their simplest form the aluminophosphate molecular sieves consist of essentially equal numbers of AlO.sub.2.sup.- and PO.sub.2.sup.+ tetrahedra and are, therefore, electrically neutral without the need for charge-balancing cations or anions. As such, these aluminophosphates do not exhibit ion-exchange properties, but nevertheless have been found to preferentially adsorb polar molecular species in a manner similar to the zeolitic aluminosilicate molecular sieves. When tetrahedral oxides of other elements such as silicon are incorporated into the structures, the aluminophosphates usually exhibit ion-exchange capacity and develop catalytically active acidic sites more characteristic of zeolites.
In the manner similar to the preparation of aluminosilicate molecular sieves, the aluminophosphate molecular sieves are synthesized by hydrothermal crystallization from aqueous reaction mixtures containing reactive precursors of PO.sub.2, AlO.sub.2 and any other metal oxide units which form the open structure of the aluminophosphate products. Although a few aluminophosphate species have been synthesized without the aid of nitrogenous organic bases, the conventional synthesis of these compositions involves the use of such bases as structure-directing materials. In addition to directing the crystallization by a templating mechanism, these nitrogenous compounds are usually strong organic bases which form complexes with the phosphorus-providing source, most commonly orthophosphoric acid, and thus are intimately involved in the chemistry of the synthesis reactions. It is sometimes the case that the nitrogenous base which actually directs the formation of the aluminophosphate product is employed in amounts only necessary for the templating function, with the requisite degree of adjustment to the reaction mixture pH being provided by supplemental, and often less expensive, organic bases.
For convenience of reference herein and for use in the attached claims, the term "molecular sieve" is intended to include the crystalline microporous aluminosilicates and aluminophosphates, but to exclude the so-called carbon molecular sieves or other non-crystalline compositions having a microporous character not determined by the location of atoms in a crystal lattice.
In the synthesis of the aforementioned molecular sieves, the templating agents are most commonly either amines or quaternary ammonium bases. The supplemental organic bases are most commonly amines. These compositions, if not relatively expensive, are usually toxic reagents. While a relatively small proportion of the templating agent exists at the end of the crystallization period as charge-balancing cations in the aluminophosphate product or simply as occluded material in the pore system of the product crystals, the bulk of the templating agent as well as the supplemental organic base in the starting reaction mixture remain in the mother liquor in one form or another. If unrecovered, these organic materials cause severe problems in the disposal of the mother liquor as waste due to its toxic nature. The high replacement costs of unrecovered organics are also a severe economic loss. It has now been discovered, moreover, that the manner and time of recovery of the organic bases can also have a marked effect upon the yield of the crystalline aluminophosphate product and the physical properties of that product.